LU-4175 tests: fix obdecho_test() to calculate write count

LU-4175 tests: fix obdecho_test() to calculate write count

This patch fixes obdecho_test() in sanity.sh to calculate
write count so as to avoid out of space failure while the
page number is large but the obd target size is small.

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: Ifcd055c587570d3f0e3ca22d763f9478e9bee3fa
Reviewed-on: http://review.whamcloud.com/8388
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>